Cathode structure for electron discharge devices



5 Sheets-Sheet l J. W. SKEHAN ET AL Filed May 26, 1945 wwwws ATTORNEYS CATHODE STRUCTURE FOR ELECTRON DISCHARGE DEVICES June 10, 1947.

June 10, 1947. Y w, sKEHAN ET AL 2,422,142

CATHODE STRUCTURE FOR ELECTRON DISCHARGE DEVICES Filed May 26, 1945 3 Sheets-Sheet 2 June 10, 1947. J. w. SKEHAN ET AL 2,422,142

CATHODE STRUCTURE FOR ELECTRON DISCHARGE DEVICES Filed May 26, 1945 'asheets-sheet 5 along the axis of the group of strands.

Patented June 10, 1947 CATHODE STRUCTURE FOR ELECTRON DISCHARGE DEVICES Joseph W. Skehan and Bror K. M. Magnusson,

Stamford, Conn., assignors to 'Machlett Laboratories, Inc., Springdale, Conn., a. corporation of Connecticut Application May 26, 1945, Serial No. 596,038

Claims.

This invention relates to cathodes for use in electrical discharge devices and is concerned more particularly with a vacuum tube containing a novel cathode structure which offers numerous advantages over prior similar structures in that it is more rugged, is simpler in construction, and is less likely to fail in service. The new cathode structure may be advantageously employed in electron tubes of various kinds, such as radio transmitting tubes, rectifier tubes, etc, and adaptations of the invention for those purposes will be illustrated and described for purposes of explanation. It is to be understood, however, and will be readily apparent that the utility of the invention is not limited to those specific applications,

In radio transmitting tubes of high power as heretofore constructed, the cathode filament structure has commonly consisted of a number of strands of tungsten wire extending parallel and arranged to define a cylinder. The individual strands are anchored at one end to fixed supports and their other ends are attached by appropriate means to one end of a rod extending The central rod is mounted on a spring, so that when the filament strands are heated and expand, the rod will be urged by the spring to maintain the strands taut and prevent them from bending out of place. It is important to prevent such distortion of the strands as a result of their expansion since, in such tubes, the filament structure is enclosed within a grid and, if an expanded filament strand makes contact with the grid during the operation of the tube, the tube is liable to be seriously damaged.

The prior arrangement described, which includes a single spring for maintaining the strands under tension when they expand upon heating, has been recognized as unsatisfactory, in that the provision of an acceptable spring is difiicult, because of the temperature conditions under which it must operate. Also, if, because of accidental conditions, some strand in the structure described remain unheated while others are heated. or some are heated less than others, the colder, less expanded strands will prevent the spring from functioning to keep the hotter strands taut, and these hot strands will then be subject to distortion. Upon the occurrence of such conditions in the filament structure referred to, there is a serious danger that the tube will be injured.

To overcome the difliculties inherent in the use of a single spring, some filament structures have been developed in which each strand is provided with its own tension spring. Such an arrangement avoids the danger of damage to the tube resulting, in the single spring structure, from that spring being rendered inefiective by the presence of one or more cold strands in the group, but increases the difficulty of providing spring means capable of functioning satisfactorily at the high operating temperatures.

The present invention is, accordingly, directed to the provision of a novel filament structure which overcomes the difliculties and avoids the disadvantages inherent in prior similar structures. The new structure includes no take-up springs, so that it is easier to manufacture than the prior structures, and it is not subject to failures in operation arising from failure or inefiectiveness of springs.

In its broadest aspect, the new filament structure may be said to be so constructed as to provide for controlled expansion of the filament strands and it includes a plurality of strands of suitable wire attached at opposite ends to fixed supports and so shaped between their ends in their cold condition, that, upon being heated, the individual strands will assume new forms in which the strands have a desired space relationship to other objects, such as a grid or other tube element. For ease in manufacture andbecause of other considerations involved in electron tube construction, the fixed supports for the strands are preferably arranged in two spaced coaxial series and the strands are helically curved. Each strand extend between a pair of supports which are offset by an angle, which is preferably although it may be greater or less than 180. The form of the individual strands and their length are then selected so that, when the strands are heated to normal operating temperatures and expand, each strand will lie throughout its length in a definite surface, such as the surface of a cylinder, about the axis of the two groups of supports. When the strands cool off, their middle portions shrink inwardly toward the axis, so that, if the structure were rotated about that axis, with the strands cold, they would define a closed surface,

which, in the specific construction under dis-- cussion, is of hour glass shape. I

The final shape that the new filament structure is to assume, when the filaments are'heated, will be determined by a number of considerations, such as space limitations within the tube, operating temperatures, the form of another tube element, etc., and the arrangement of the fixed supports and the shapes of the filaments, when cold, between those supports will be selected in I dependent, as in prior structures employing a single take-up spring, upon the condition of other strands in the group, it i apparent that'the new structure may be safely operated, regardless of the number of strands that are heated at any particular time Also, since the new structure is not dependent, either for satisfactory performance or for long life, upon the proper functioning of springs, it-is more rugged and of greater durability than prior structures in which spring take-up means are used. Similarly, the use in the new structure of fixed supports, to which the strands are securely attached, simplifies fabrication and reduces the cost, because of the elimination of parts.

For a better understanding of the invention, reference may be had -to the accompanying drawings, in which Fig. 1 is a longitudinal cross-sectional view of a three element tube in which the new cathode structure is employed;

' Fig. 2 is an enlarged view in side elevation and with parts broken away of one of the end supports of the cathode structure shown in Fig. 1;

Fig. 3 is a sectional view on the line 33 of. Fig. 2;

Figs. 4 and 5 are sectional views on the lines s-& and 5-5, respectively, of Fig. I;

Fig. 6 is a view in perspective of the new filament structure in cold condition;

Fig. 7 is a. view similar to Fig. 6 but showing the filaments heated and expanded;

Fig. 8 is a, fragmentary longitudinal sectional view of a rectifier tube employing the cathode structure of the invention, the filament of the tube being illustrated in heated condition; and

Fig. 9 i an end view of the supports for the outer ends of the filaments of the rectifier tube of Fi 8.

In Fig. 1, the new cathode structure is illustrated as employed in a typical radio power transmitting tube which comprises a glass envelope iii, to one end of which is sealed at i i a tubular metal section I2 closed at its outer end and forming the anode or plate of the tube. The anode is provided with an internal tubular extension i3 which projects rearwardly into the glass envelope past the metal-glass seal l I to protect the seal against electron bombardment. A ring l2a encircling the anode serves as part of the tube mounting.

At the end opposite to that on which the anode is mounted, the envelope has a re-entrant end portion generally designated i4, and including a skirt I 5 in the free edge of which is sealed a cylindrical metal section i6 secured to a similar section I! by opposed edge flanges I8 on the sections. A plurality of supporting rods ii! are secured to section I! and extend forwardly therefrom. Rods I! pass through and are secured to the inner surface of a conical metal shield and the forward ends 2| of rods l9 are exposed beyond the forward end of the shield. Wires 22 forming a grid are wound about the exposed end portions 2| of rods IS. A conductor 23 sealed through the wall of a neck 24 projecting from the side of envelope I0 is connected to flanges I8 and serves as the lead by which voltage is maintained von th grid in the operation of the tube.

A plurality of metal supporting rods 25 are sealed through the re-entrant end wall of the tube and extend forwardly through the sleeve 28 and the interior of grid 22. The rods are kept in proper spaced relation by a pair of blocks 26,

' 21 of insulating material having openings through which the rods extend, block 21 being disposed within the end of sleeve 20. In the construction shown, there are six rods 25 spaced angularly so as to lie in the surface of a cylinder, and a supporting rod 28 mounted at one end in blocks 26, 21 lies in the axis of the cylinder. At its outer end, rod 28 carries a pair of brackets 29 which are secured to the bottom of a metallic cup 30. This cup is formed with pairs of slots extending inwardly from its rim and defining tongues 3 I which lie equally spaced about the rim of the cup. The slots are slightly inclined to the axis of rod 28, as indicated in Fig. 2.

Each of rods 25 is connected by a coupling 32 outside the envelope to a conductor 33. At its inner end, each rod has an inclined end portion 3d, to which is attached one end of a filament 35. To make the connection, the end portion 36 of each rod is formed with a flat to which the end of the filament is secured by a wrapping 36 of molybdenum or other suitable wire, after which the parts are welded together.

Each of filaments 35 is connected at its outer end to one of the tongues 3! by being welded thereto, and is further held in place by a wrapping 31 of wire about the end of the filament and the tongue. The cup 3B is so mounted that the tongues 35 thereon are generally aligned with respective rods 25, and the tongue 3!, to which a filament 35 is rigidly connected at one end, is oifset from the end portion of the rod 25, to which the other end of the filament is rigidly connected. Each filament is thus of generally helical form, but the length of filaments is such that, in the cold condition of the filaments, the mid-portions 35a lie inward from the surface of 'a cylinder passing through rods 25 and tongues 3 I. The formation of the individual filaments is, therefore, such that if the assembly were rotated on the axis of rod 28 with the filaments in cold condition, they would define a closed surface of hour glass form. When the filaments are heated and expanded, as shown in Fig. 7, their mid-portions 35a move outwardly so as to lie in the surface of the cylinder passing through rods 25 and tongues 3i.

It will be noted that, in the tube shown in Fig. 1, the grid is spaced relatively closely to the filament assembly, and it is, therefore, important that, when the filaments are heated, they do not contact with the grid. Also in such a tube, it is desirable that the spacing of all parts of the filaments from the plate be uniform. By forming the filaments so that in their cold condition, their mid-portions lie inward from a cylinder passing through rods 25 and tongue 3!, it is possible to insure that when the filaments expand, they will be properly spaced throughout their lengths from the grid and will have the proper relation to the plate.

In the tube shown in Fig. 1, the cup 30 constitutes a common connection for all the file.- ments, and the six rods 25 serve as leads, by which various connections can be made for supplying energy to the filaments. Thus, when a single phase source is employed, three filaments may be connected in parallel to each side of the supply. When a three-phase source of energy is employed,

' two filaments may be connected to each terminal in a star connection. When a six-phase source in the tube of Fig. 1, it will be apparent thatother numbers of filaments may be used, if desired.

The tube shown in Fig. 8 is another typical tube, in which the new filament structure may be used to advantage. The tube is a rectifier tube, and it includes a glass envelope 38 having a re-entrant end portion 39 to the end of which is sealed a metallic cup 40 lying within a shield 4|. Supporting rods 42 are attached to cup 40 and at their inner ends, the rods support a tubular anode 43.

At it other end, the tube has a re-entrant portion 44, through which are sealed conductive rods 45 arranged at the corners of a square. The rods 45 project into the free end of anode 43, and within the group of rods 45 are a pair of longer rods 46 also sealed through the end of the envelope. Rods 46 are formed with ends 41 bent into hook shape and lying in a plane transverse to the axis of the'anode. The free ends of the hook-shaped portions of rods 46 extend therefrom in the same direction, and each hook-shaped end overlies the ends of a pair of rods 45.

A filament 48 is attached to the end of each rod 45 by being welded thereto and wrapped, as indicated at 49, and each filament is attached to the hook-shaped end 41 of one of the rods 46. The point of attachment of the outer end of each filament lies offset 180 from the end of the rod 45, to which it is attached at the other end, and the filaments are thus of generally helical formation.

In the cold condition of the filaments, the midportions thereof 'lie inward from a cylinder passing through the points of attachment of the filaments to rod 45 and the hook-shaped ends of rods 66. Accordingly, if the filament assembly were rotated about the axis of anode 43 with the filaments in cold condition, they would define a closed surface of hour glass form. When the filaments are heated and expand, their mid-portions bulge outwardly so as to lie in a cylinder passing through their points of support. In the heated condition of the filaments, therefore, they are uniformly spaced from the anode 43 throughout their entire lengths, whereas, if the filaments were initially of the form shown in Fig. 8, they would, upon heating, bulge outwardly, so that their mid-portions would lie nearer to the anode than their points of support.

In both forms of the new cathode structure, each filament is attached at each end to a fixed support, and, when the filaments are heated and expand, they are subjected to compression rather than tension, as in the prior constructions, in which the filaments are maintained taut by being acted on by spring take-up means.

In the forms of the cathode structure illustrated and described, the filaments have their respective ends offset by 180, but this specific angle of offset is not essential and the angle varies inversely with the diameter of a cylinder passing through the points of support of the filaments and directly with the length of such a cylinder. Thus, in a structure, in which the diameter of the cylinder is relatively small and the length of the cylinder is relatively great, the amount of shortening of the individual filaments obtained by forming them so that, in their cold condition, their central portions lie inward from the surface of the cylinder, may not be suflicient to compensate for their expansion when they are heated. In such a structure, it may be necessary to offset the ends of a filament by an angle of more than 180 and, in extreme cases, by an angle of 360 or more. However, it is preferable, where possible, to utilize an angle of offset of less than 360, selecting such an angle and so forming the filatubes of different constructions.

In the foregoing, we have described the filament structure as including a plurality of filaments, but the invention may be utilized in a structure containing a single filament. Such a singlefilament structure may be desirable under specific conditions.

We claim:

1. In a vacuum tube, a cathode including fixed supports disposed in spaced relation in two groups lying remote from one another, the supports in the respective groups lying in circular series in substantially parallel planes, and a plurality of filaments each connected rigidly to a pair of supports one in each group, the supports of each such pair being offset angularly from one another by an angle of not to exceed 360, the filaments being'of generally helical formation and of such form in cold condition, that the portions of the filament between their ends lie inward from a closed surface passing through the supports in the two series.

2. In a vacuum tube, the combination of a cathode structure and another tube element enclosing a, portion of the cathode, the cathode structure including fixed supports disposed in spaced relation in two groups lying remote from one another, and a plurality of filaments lying within the other tube element and each connected rigidly to a pair of supports one in each group, which are angularly offset by an angle of the order of 180", the filaments being of generally helical formation and of such form in cold condition that, between their ends, the filaments lie inward of a substantially cylindrical surface passing through the supports.

3. In a vacuum tube cathode structure, a plurality of fixed supports angularly spaced about a common central axis and arranged in two groups remote from one another, the supports in each group lying tangent to a circle and said circles lying in substantially parallel planes, and a plurality of filaments, each rigidly secured to a pair of angularly offset supports one in each group, the filaments being of generally helical form and the portions between their ends lying inward from a closed surface passing through the supports in the two groups, when the filaments are cold.

4. In a vacuum tube cathode structure, a group of filaments extending generally helically about a common longitudinal axis, a plurality of supports arranged in two groups spaced along said axis, the supports in each group lying substantially in a cylindrical surface about said axis, each filament being rigidly attached at its ends to a pair of supports, one in each roup, which are angularly offset, the portions of the filaments between their ends lying inward from said cylindrical surface, when the filaments are cold, by an amount such that, when the filaments are raised to normal operating temperature, the expansion of the filaments causes them to lie in said surface throughout their lengths.

5. In a vacuum tube cathode structure, a group of filaments rigidly supported at their ends only and extending generally helically through an angle of about 180 about a common longitudinal axis, the ends of the filaments lying substantially in a cylindrical surface about said axis and the portions of the filaments between their ends lying inward from said surface, when the filaments are cold.

6. In a vacuum tube, a cathode including a pair of fixed supports in spaced relation lengthwise of the axis of the cathode, said supports lying outwardly from and angularly oflset about said axis, and a filament extending generally helically about said axis through an angle of not to exceed 360 and connected at its ends rigidly to the supports, the portion of the filament between its ends lying inward from a closedsurface about said axis through said supports, when the filament is cold.

7. In a vacuum tube, a cathode including a pair of fixed supports in spaced relation lengthwise of the axis of the cathode and lying outwardly from said axis and angularly ofiset about said axis by an angle of not to exceed 360", and a filament wnnected at its ends rigidly to the supports, the filament being of generally helical formation and of such form in cold condition that its mid-portion lies inward from a closed surface about said axis through said supports.

8. In a vacuum tube cathode structure, a pair of fixed supports angularly spaced about an axis and also spaced from one another lengthwise of said axis and lying outward from said axis, and a filament rigidly connected at its ends to the supports, the filament extendin generally helically about said axis through an angle of not to exceed 360 and being of such form in the cold condition, that its mid-portion lies inward from a cylindrical surface about said axis and passing through said supports.

9. In a vacuum tube cathode structure, a pair of fixed supports angularly spaced about an axis and also spaced from one another lengthwise of.

said axis and lying outward from said axis, and

a filament rigidly connected at-its ends to the supports, the filament extending generally helically about said axis through not to exceed one convolution and being of such form, when cold, that its mid-portion lies inward from a cylindrical surface about said axis passing through said supports by an amount such that, uponexpansion of the filament when it-is raised to normal operating temperature, it lies substantially in said surface throughout its length.

10. In a vacuum tube cathode structure, a pair of fixed supports lying remote from one another lengthwise of an axis and lying-outward from said axis, and a filament rigidly connected at its ends to the supports and extending generally helically about said axis through an angle of about the filament being so formed that, when it is cold, its mid-portion lies inward from a cylindrical surface about said axis and passing through the supports.

JOSEPH W. SKEHAN. BROR K. M. MAGNUSSON.

REFERENCES CITED The following references are of record in file of this patent:

UNITED STATES PATENTS the &

Holland Aug. 15, 1939 

